A super luminescent diode is a light emitting device capable of providing a relative high optical output of 1 mW or higher like a semiconductor laser while having a wideband spectrum distribution like a light emitting diode. Such a super luminescent diode will be called an SLD in some descriptions below. The SLD have received attentions in medical fields and measurement fields because of its characteristics and may be used as a light source in optical coherence tomography (OCT) capable of acquiring a tomogram of a biological tissue, for example. As a light source in OCT, an SLD may be applicable which has a wide light emission wavelength band for a higher depth resolution. According to NPL 1, in order to increase a light emission wavelength band of an SLD, a single quantum well structure is used which has a plurality of different energy levels of an emission spectrum as an active layer. Electric current to be injected to the active layer of the SLD may be increased to produce light emission with a ground level corresponding to emitted light having a long wavelength and light emission with a high-order level (primary level) corresponding to emitted light having a short wavelength and thus achieve an emission spectrum having a half-width of 48 nm.